The present invention relates to optical instruments such as process cameras, electrostatic copying machines. More particularly, it relates, in an optical instrument constructed to support a sheet material such as a sensitive material or an original film on a planar transparent plate, to a device for pressing and contacting said sheet material with said transparent plate.
Typical optical instruments adapted to support a sheet material on a transparent plate having a horizontal plane includes a slit exposure type camera as shown in FIG. 7. Such camera, taken as an example, will be described below.
The optical system thereof is so arranged that light from a film 2 on a horizontal original mount 1 equipped with a vacuum suction mechanism travels via a movable optical system consisting of a triangle mirror 3, a lens 4, and a mirror 5 and then through a transparent plate 6 to reach a photosensitive material 7. When the original film 2 is manually set on the original mount 1, a sensitive material 7, with its sensitive surface down, is nipped by rollers 8 and carried in onto a predetermined position on the transparent plate 6 by suction transfer means 9, whereupon a contact device 20 shown in FIG. 8 (to be later described in more detail) is lowered to press the sensitive material 7 from behind to contact the latter with the transparent plate 6. In this condition, scanning exposure is effected by moving said optical system, whereupon the sensitive material 7 is carried out by the suction transfer means 9 and carry-out rollers 11.
In this connection, in such camera, if there is a clearance between the sensitive material 7 and the transparent plate 6 during exposure, this results in incorrect focussing and other inconveniences. For this reason, use is made of the contact device 20 adapted to press the sensitive material 7 from behind to contact it with the transparent plate 6. FIG. 8 is a perspective view of a conventional contact device 20 from below. In this figure, a frame 21 is vertically movable by rotating eccentric cams 24 and 25 mounted on drive shafts 22 and 23, and the frame assumes its upper position when the sensitive material 7 is moving and its lower position during exposure. When the frame 21 is in its lower position, a rubber plate 27 bonded to an L-shaped member 26 fixed to said frame 21 presses the sensitive material 7 from behind to contact it with the transparent plate 6. In addition, said frame 21 is locally formed with openings to allow movement of the suction transfer means 9.
The conventional contact device 20, which is designed to contact the sensitive material 7 by means of a rigid body such as said L-shaped member 26, as described above, has the following inconveniences.
It is difficult to attain planar alignment between the transparent plate 6 and the L-shaped member 26, and its machining takes much time and even if it is machined with high accuracy, it locally fails to contact. Further, since the middle portion of the transparent plate 6 is downwardly deflected under its own weight, there is a difference between the degrees of contact in the middle and peripheral regions. Further, the simultaneous pressing of the entire surface of the sensitive material can result in a small air pool being formed in the middle or in a movement of the sensitive material due to the flow of the air held between the sensitive material 7 and the transparent plate 6 when said air escapes from therebetween, said movement resulting in the sensitive material 7 being contacted with the transparent plate 6 at a position deviated from the predetermined position. A further inconvenience is that a locally curled sensitive material 7 is caused to bend or wrinkle or is moved to contact at a deviated position.